auto-analyze is slow due to hardcoded concurrency 1

[chrysan]

Enhancement

auto-analyze concurrency is now hardcoded to be 1 which causes auto-analyze runs too slowly and may fail due to exceeding gc-life-time at versions before v6.2

tidb/executor/adapter.go

Lines 120 to 135 in 3507a72

	if _, ok := a.plan.(*plan.Analyze); ok && ctx.GetSessionVars().InRestrictedSQL {
	oriStats := ctx.GetSessionVars().Systems[variable.TiDBBuildStatsConcurrency]
	oriScan := ctx.GetSessionVars().DistSQLScanConcurrency
	oriIndex := ctx.GetSessionVars().IndexSerialScanConcurrency
	oriIso := ctx.GetSessionVars().Systems[variable.TxnIsolation]
	ctx.GetSessionVars().Systems[variable.TiDBBuildStatsConcurrency] = "1"
	ctx.GetSessionVars().DistSQLScanConcurrency = 1
	ctx.GetSessionVars().IndexSerialScanConcurrency = 1
	ctx.GetSessionVars().Systems[variable.TxnIsolation] = ast.ReadCommitted
	defer func() {
	ctx.GetSessionVars().Systems[variable.TiDBBuildStatsConcurrency] = oriStats
	ctx.GetSessionVars().DistSQLScanConcurrency = oriScan
	ctx.GetSessionVars().IndexSerialScanConcurrency = oriIndex
	ctx.GetSessionVars().Systems[variable.TxnIsolation] = oriIso
	}()
	}